I would guess that the finish at the back edge of the exterior case is overlapping the edges of the interior case. That's why Danlib1 cautioned to be careful in chipping the finish. If you really want to get the case open you may have to use an Xacto knife or something similar to trim back the finish beyond the edges of the interior case.
I think the Decco I have is a great sounding integrated with a lot of flexibility but part of the reason it's so reasonably priced is that the construction isn't first-rate. I expect the same thing is true of the Nova.